The present invention relates to a solid-state image sensor, and a manufacturing method thereof.
Prior art in the field will be described, referring to the drawing.
FIG. 1 is a view of an example of a circuit of a solid-state image sensor called an amplifying MOS sensor.
In FIG. 1, amplifying transistors 2 for reading a signal from a photodiode 1 and 3.times.3 unit cells comprising a reset transistor 3 for resetting a signal charge are arranged in a two-dimensional form. In reality, more unit cells than those shown in FIG. 1 are arranged. Horizontal address lines 5 extending in the horizontal direction from a vertical shift resistor 4 are connected to the gates of vertical select transistors, and specify the line for reading signals. Reset lines 6 are connected to the gates of the reset transistors. The sources of the amplifying transistors 2 are connected to vertical signal lines 7, and the one end of each of the lines 7 is connected to a load transistor 8. The other thereof is connected to a horizontal signal line 11 through a horizontal select transistor 10 to be selected by a select pulse supplied from a horizontal shift resistor 9.
In a conventional method for forming a photodiode 1 in a photoelectric conversion section (see FIG. 2), a p-well (p-type layer) 12 is formed on a Si substrate 13, and a resistor 14 is applied onto the Si substrate at other than the portion where the photodiode 1 is to be formed, to carry out patterning. Subsequently, phosphorus 15 (P) is implanted thereto by ion implantation at one time. After that, thermal treatment is conducted to form the photodiode 1. The distribution of impurity concentration is shown in FIG. 3.
FIG. 3 shows the distribution of phosphorus (P) concentration, which is for forming the photoelectric conversion section, the distribution of boron (B) concentration, which is for forming the p-well, and the distribution of net impurity concentration. Such a sort of device has the following disadvantages:
(1) Carriers in a sufficient amount which are generated by photoelectric conversion cannot be stored, since the capacity of the photodiode 1 is small.
(2) Carrier generated in the deep position in the substrate by photoelectric conversion leak into the photodiodes for adjacent pixels so as to result in mixed colors.
(3) The area for collecting the carrier in the photodiode is small and consequently sensitivity is low.
In the case of implanting a great deal of an ion in the conventional method, ion damage is concentrated into a certain depth (for example, near 0.2 .mu.m) so that dark current increases.
As described above, the conventional MOS-type solid-state image sensor has the disadvantages of small capacity of the photodiode, mixed color, low sensitivity and generation of a dark current in a large amount.